Method of forming chromium diffusion coatings



United States Patent ice 3,186,865 METHOD OF FGRMING CHRGMIUM DUSIGN CGATINGS John P. Page, Thousand (Bake, Caiifi, assignor to North American Aviation, inc. No Drawing. Filed Got. 16, 1962, Ser. No. 231,03? 7 Claims. (Cl. i17119) My invention relates to an improved method of forming chromium-containing alloys on base metal surfaces, and more particularly to an improved method of providing an oxidation and corrosion resistant chromium surface on ferrous metals.

The industrial requirements for metals of high oxidation resistance are rapidly increasing for applications in petroleum refineries, chemical processing, aerospace applications, anti-smog devices, and the like, too numerous to mention. Since the surface properties of a metal are largely determinative of its oxidation and corrosion resistance, methods have been sought to improve such surface properties through metal coatings and alloying. Improving the surface behavior of carbon and low alloy steels, for example, would achieve economies over the use of the more costly stainless steels.

Chromium is widely used to improve the surface properties of ferrous metals and refractory metals, such as molybdenum. Chromium is generally applied by socalled pack cementation methods, wherein a base metal part is placed in a high temperature jig containing a powdered chromium composition, an inert filler, and a halide salt. Upon heating, chromium halide vapors form which deposit upon the part, forming a chromium coating thereon. Among the drawbacks of such processes are difliculties in coating complex shapes, particularly objects with hidden recesses or bores. The parts must be cleaned prior to use, and time is consumed in packing the part and the chemicals in the jig, and then later in unloading the assembly, which operations increase the cost of such processes. Further, pack methods have been generally restricted to rather small parts and are not utilizable for large, finished articles because of equipment limitations. The pack methods are also not adapted to depositing chromium alloys of high corrosion resistance, for example chromium-nickel, which on a ferrous metal base would form an Fe-Cr-Ni alloy, or in other words, a stainless steel-type jacket on a low alloy or carbon steel base.

Accordingly, an object of my invention is to provide an improved method of applying chromium-containing diffusion coatings upon a dissimilar base metal.

Another object is to provide a method of applying chromium upon a ferrous metal base to improve the oxidation and corrosion resistance of such base metals.

Another object is to provide a corrosion resistant coating of chromium upon ferrous metal objects of complex or irregular configuration, forming a diffusion layer of an intermetallic compound or alloy on the surface of the object.

Still another object is to provide a relatively economic and rapid method of forming such protective chromiumcontaining surfaces.

Yet another object is to provide a method of forming a highly corrosion resistant Fe-Qr-Ni alloy upon a low alloy or carbon steel base.

Other objects and advantages of my invention will become apparent from the following detailed description and the appended claims.

In accordance with my present invention a base metal may have a chromium-containing difiusion coating applied by placing said metal in a molten lithium bath under an inert atmosphere, said bath being provided with chromium dissolved therein, and maintaining said base 3,186,865 Patented June 1, 1965 metal in said bath until a chromium layer of the desired thickness is obtained.

I have surprisingly discovered that high quality, bright, chromium-rich diffusion layers, covering complicated shapes with recesses or bores, may be achieved from a molten lithium bath. The chromium dissolved in the lithium bath diiiuses therethrough onto the base metal. The use of lithium is critical; no other alkali metal is alone operable. The lithium may be diluted by another alkali metal, such as sodium, which itself is not operable, but which is considerably cheaper. However, the vapor pressure of the other alkali metals limits the practical lithium dilution factor, due to the temperature requirements of my process, described below, and to the desire to use ambient pressures.

While the mechanism of the reaction is not completely understood, there is actual difiusion of the chromium into the base metal, forming an alloy or intermetallic compound of graduated composition, chromium rich at the surface and leaner toward the interior. Such alloy formation greatly improves the quality and adherence of the coating. For example, with ferrous metal bases, a chromium-iron alloy is formed. Thus, the oxidation and corrosion resistance of such coated ferrous metals is very great, while the cost is considerably less than of stainless steel.

The present process is relatively simple and economical to perform. The base metal need not be cleaned nor arranged into retorts with the reagent chemicals in a timeoonsuming operation, as in the pack methods. Further, since there is no theoretical limitation to the size of the lithium bath, large, finished products may be chromized. The resulting surface is also ductile, and the products may be worked and welded.

Turning now to the details of the present process, the molten lithium bath is maintained at an elevated temperature, at least about 1500 F.; the maximum temperature is set by the boiling point of lithium at any given pressure. Lithium boils at about 2400 F., and therefore the practical temperature range is about 15002400 F. A temperature of about 1800-2200 F. is particularly useful, and a temperature of about 2000- F. is generally optimum. The lithium is contained in a metal which is stable to lithium; a number are available, for example low alloy steel, stainless steel, and columbium. The bath is maintained under an inert gas atmosphere, for example, a noble gas such as helium, or nitrogen, since lithium will react with air. q

The concentration of chromium in the lithium bath may satisfactorily vary, and a particular concentration is not critical. A larger concentration increases the difusion rate and hence the plating rate. Further, the surface area of the target, the possible plating on container surfaces, and the need for a reservoir dictate greater than saturation concentrations, in order to maintain satu-' rated lithium throughout a plating run. For example, about 25 weight percent chromium is satisfactory in a lithium bath maintained at a temperature of about 2000 F.

The time required for coating a base metal is a function of the temperature of the bath, the concentration of chromium, and the coating thickness desired. .The coating thicknesses will vary over a range of about 0.1-30 mils, while a chromium d-iliusion layer of about 2-5 mils may be considered typical. It takes approximately 5 hours at 2000 F. to apply a coating of about 35 mils from a saturated solution of chromium. Under similar circumstances a coating of about 0.1 mil can be applied in about 30 minutes, and a 10-15 mil coating will require about 24 hours. It will take about hours to eposit less than 1 mil of chromium from. a bath containing a saturation concentration of chromium at l500 F., while the same thickness of chromium can be deposited from a bath maintained at about 2400" F. in one hour or less.

The chromium is dissolved in the bath very simply by addition in granular metal form.

In addition to forming protective chromium layers on base metals, I find that highly corrosion resistant surfaces can be obtained by additionally providing nickel in the bath. Nickel alloys with the chromium on the base surface and forms an Fe-Cr-Ni alloy on ferrous metals. The concentration of nickel and chromium in the alloy formed on the surface of such ferrous metal-s will meet or even exceed the concentrations of these metals in stainless steels, for example about 22 weight percent Cr and 24 weight percent Ni, and hence high corrosion resistance is obtained. Chromium-nickel alloys apparently cannot be formed on metal surfaces by prior art pack methods and this may partly account for, in addition to the satisfactory results obtained with chromium alone, the absence of nickel-chromium alloy coatings in such prior industrial practice. As an example of comparative oxidation resistance, a carbon steel specimen heated in static air at 1500 F. gained 0.1 gram in only hours, While a similar specimen coated with chromium according to my process required 600 hours for the gain of 0.1 gram in weight, and the Cr-Ni coating required 2000 hours for the same weight gain. Nickel in granular or chip form readily dissolves in the lithium. While the concentration of the nickel in the bath may be adjusted according to the concentration desired in the final product, diffusion rates are greater with higher concentrations, and above saturation concentrations are generally preferred, as with chromium. For example, about 25 weight percent is found to be highly satisfactory in a lithium bath maintained at about 2000 F.

In addition to forming Cr-Ni diffusion coatings, other complex Cr-containing alloys may be formed. Such alloying elements as Al, Si, Ti, C, and Cb may be added to the bath with the chromium in the manner heretofore described, and such diffusion coatings as Fe-Cr-Al may be obtained on ferrous metals and Cb-Cr-Al on columbium.

The following examples are offered to illustrate my invention in greater detail.

EXAMPLE I Chromium Twenty grams of lithium, 5 grams of metallic chromium, and four /2" x 1" x 0.062" tabs of low carbon steel were placed in a 6 schedule 40 carbon steel capsule which Was sealed by Welding As"-thick carbon steel end caps in an argon atmosphere. The capsule was placed in a tube furnace at 2000 F. for 5 hours during which time the capsule was rotated at revolutions per minute with the axis of the capsule at 30 to horizontal.

After opening the capsule and removal of the lithium from the surfaces of the tabs, the following experiments were performed and results obtained.

(1) One of the tabs was inserted in 50% nitric acid. No

reaction was observed.

(2) A small section was cut from this tab and mounted for metallographic examination. A Cr alloy case depth of approximately 3.5 mils was noted.

(3) Another section of the same tab was bent and placed in nitric acid. The carbon steel which had been exposed by sectioning reacted vigorously with the acid, but the case remained unattacked.

(4) This case Was subsequently analyzed to contain 23.89% Cr. and 76.02% Fe.

(5) A second tab was reduced in thickness 50% by rolling; it remained resistant to nitric acid. A slight attack by nitric acid was observed after an overall re duction in thickness of about 80%.

(6) A third tab was cut in half, then re-joined by tungsten electrode, inert-gas welding using a stainless steel filler rod. This specimen was resistant to nitric acid after welding.

(7) The fourth tab was placed in a furnace at 1500" F. This specimen gained about 1 mg./cm. in 400 hours, but gained about 9 more mg./cm. in the next 1 50 hours, at which time the test was terminated. As a comparison, uncoated carbon steel gains about 60 mg./cm. in only 40 hours at 1500 F.

EXAMPLE II Chromium-nickel to the lithium. The following results were obtained.

( 1) One tab was resistant to nitric acid.

(2) A coating thickness of 2 mils was measured metallographioally.

(3) Analysis of the coating indicated its composition to be 21.98% Cr, 24.3% Ni, and 54.7% Fe.

(4) A coated tab suspended in a furnace at 1500 F. gained 10 mg./cm. in about 1300 hours (extrapolated from 0-hour datum point).

EXAMPLE III Sodium bath A capsule Was prepared as in Example I except that sodium was used in place of lithium. The capsule was heated at 2000 F. for 5 hours. No visible coatings on the mild steel specimens were obtained.

A capsule containing a mixture of 50 weight percent lithium and 50 weight percent sodium was next prepared as above, and was treated at 2000 F. for 5 hours. A chromium coating which appeared substantially similar to the coating from the all-lithium capsules was obtained.

The above examples are illustrative rather than restrictive of my invention, which should be understood to be limited only as is indicated in the appended claims.

I claim:

1. A method of forming a chromium-containing alloy 011 a chromium-alloyable base metal which comprises pro viding a molten bath containing lithium as active solvent under an inert gas atmosphere, dissolving chromium in said bath, placing said base metal in said bath, and maintainin g said base metal in said bath at a temperature of approximately 15002400 F. until a coating of chromium is obtained thereon.

2. The method of claim 1 wherein said bath contains up to 50 percent by weight of sodium.

3.'The method of claim 1 wherein said base metal 7 is a ferrous metal.

4. A method of forming a chromium diffusion alloy on the surface of a ferrous metal which comprises providing a molten bath containing lithium as active solvent maintained at a temperature of approximately 1800- 2200" F. under an inert gas atmosphere, dissolving chromium therein, placing said ferrous metal in said bath, maintaining said ferrous metal in said bath for a period of approximately /;--30 hours, thereby forming an Fe-Cr alloy of graduated composition on the surface of said ferrous metal.

5. The method of claim 4 wherein said bath is maintained at a temperature of approximately 2000 F., approximately 25 weight percent chromium is provided in said bath, and said ferrous'metal is kept in said bath for approximately 5 hours.

6. A method of forming an Fe-Cr-Ni alloy protective surface on a ferrous metal base, which comprises providing a bath containing lithium as active solvent maintained at a temperature of about 15002400 F. under an inert gas atmosphere, dissolving chromium and nickel in F t a said bath, placing said ferrous metal base in said bath, References Cited by the Examiner and maintaining said base metal in said bath until said UNITED STATES PATENTS Fe-Cr-Ni alloy is formed.

7. A method of forming a chromium-nickel diffusion 2848352 8/58 Noland et a1 1171 14 X H 1 u t 61 b hi h r 5 3,085,028 4/63 Logan 117-114 W a P a W. s W 5 P 3,086,886 4/63 Kietfer et a1 117 114 vidmg a batn contammg lithium as active solvent mamtained at a temperature of about 1800-2200 F., under POREKGN PATENTS an inert gas atmosphere, dissolving at least saturation 613,224 7 /62 Belgium concentrations of nickel and chromium in said bath, and placing said base metal in said bath until a protective Fe- 10 RICHARD NEVIUS, Primary Examiner. Cr-Ni alloy is obtained on the surface thereof. WILLIAM D, MARTIN, Examiner,

erse Decision in Interference Patent, No. 3,186,865 J. P. Page,

Notice of Adv DIFFUSION commas, final ce No. 95,105 involving In Interferen FORMING CHROMIUM lered May 1?), 1968, as To (flaim 3.

METHOD OF judgment adverse to the patentee was renc Demmbw 17', 1968.] 

1. A METHOD OF FORMING A CHROMIUM-CONTAINING ALLOY ON A CHROMIUM-ALLOYABLE BASE METAL WHICH COMPRISES PROVIDING A MOLTEN BATH CONTAINING LITHIUM AS ACTIVE SOLVENT UNDER AN INERT GAS ATMOSPHERE, DISSOLVING CHROMIUM IN SAID BATH, PLACING SAID BASE METAL IN SAID BATH, AND MAINTAINING SAID BASE METAL IN SAID BATH AT A TEMPERATURE OF APPROXIMATELY 1500-2400*F. UNTIL A COATING OF CHROMIUM IS OBTAINED THEREON. 